Mammalian Endogenous Cannabinoid System
The various elements of the mammalian endogenous cannabinoid system (ECS) constitute a variety of pharmacological targets for the broad group of compounds generally termed as cannabinoids. Included among these elements are two types of G-protein-coupled membrane receptors: the central CB1 receptors (Matsuda, L. A.; Lolait, S. J.; Brownstein, M. J.; Young, A. C.; Bonner, T. I. Structure of a Cannabinoid Receptor and Functional Expression of the Cloned cDNA. Nature 1990, 346, 561–564); and the peripheral CB2 receptors (Munro, S.; Thomas, K. L.; Abu-Shaar, M. Molecular Characterization of a Peripheral Receptor for Cannabinoids. Nature 1993, 365, 61–65).
Also included among the elements of the ECS are the endogenous ligands anandamide (Devane, W. A.; Hanu{hacek over (s)}, L.; Breuer, A.; Pertwee, R. G.; Stevenson, L. A.; Griffin, G.; Gibson, D.; Mandelbaum, A.; Etinger, A.; Mechoulam, R. Isolation and Structure of a Brain Constituent That Binds to the Cannabinoid Receptor. Science 1992, 258, 1946–1949), 2-arachidonoylglycerol (Sugiura, T.; Kondo, S.; Sukagawa, A.; Nakane, S.; Shinoda, A.; Itoh, K; Yamashita, A.; Waku, K. 2-Arachidonoylglycerol: a Possible Endogenous Cannabinoid Receptor Ligand in Brain. Biochem. Biophys. Res. Commun. 1995, 215, 89–97), and the recently reported 2-arachidonyl glyceryl ether (Hanu{hacek over (s)}, L.; Abu-Lafi, S.; Fride, E.; Breuer, A.; Vogel, Z.; Shalev, D. E.; Kustanovich, I.; Mechoulam, R. 2-Arachidonyl Glyceryl Ether, an Endogenous Agonist of the Cannabinoid CBI Receptor. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 3662–3665). A mechanism for the termination of the biological activity of the endogenous ligands has been elucidated, composed of a carrier-mediated transport system (anandamide transporter (AT)) and a hydrolyzing enzyme, named fatty acid amide hydrolase (FAAH). Hillard, C. J.; Edgemond, W. S.; Jarrahian, A; Campbell, W. B. Accumulation of N-Arachidonoylehanolamine (Anandamide) into Cerebellar Granule Cells Occurs via Facilitated Diffusion. J. Neurochem. 1997, 69, 631–638; Beltramo, M.; Stella, N.; Calignano, A.; Lin, S. Y.; Makriyannis, A.; Piomelli, D. Functional Role of High-Affinity Anandamide Transport, as Revealed by Selective Inhibition. Science 1997, 277, 1094–1097; Hillard, C. J.; Jarrahian, A. The Movement of N-arachidonoylethanolamine (Anandamide) across Cellular Membranes. Chem. Phys. Lipids 2000, 108, 123–134; and Ueda, N.; Puffenbarger, R. A.; Yamamoto, S.; Deutsch, D. G. The Fatty Acid Amide Hydrolase (FAAH). Chem. Phys. Lipids 2000, 108, 107–121.
Importantly, the ECS seems to be involved in the regulation of a wide variety of central and peripheral processes, such as anti-nociception, brain development, retrograde neuronal communication, memory, appetite, psychomotor control, cardiovascular and immune regulation, and cellular proliferation. See (a) Calignano, A.; La Rana, G.; Giuffrida, A.; Piomelli, D. Control of Paul Initiation by Endogenous Cannabinoids. Nature 1998, 394, 277–281; (b) Walker, J. M.; Hohmann, A. G.; Martin, W. J.; Strangman, N. M.; Huang, S. M.; Tsou, K The Neurobiology of Cannabinoid Analgesia. Life Sci. 1999, 65, 665–673; (c) Fernández-Ruiz, J.; Berrendero, F.; Hernádndez, M. L.; Ramos, J. A. The Endogenous Cannabinoid System and Brain Development. Trends Neurosci. 2000, 23, 14–20; (d) Wilson, R. I.; Nicoll, R. A.; Endogenous Cannabinoids Mediate Retrograde Signaling at Hippocampal Synapses. Nature 2001, 410, 588–592; (e) Hampson, R. E.; Deadwyler, S. A. Cannabinoids, Hippocampal Function and Memory. Life Sci. 1999, 65, 715–723; (f) Di Marzo, V.; Goparaju, S. K; Wang, L.; Liu, J.; Bátkai, S.; Járai, Z.; Fezza, F.; Miura, G. I.; Palmiter, R. D.; Sugiura, T.; Kunos, G. Leptin-Regulated Endocannabinoids Are Involved in Maintaining Food Intake. Nature 2001, 410, 822–825; (g) Giuffrida, A.; Piomelli, D. The Endocannabinoid System: a Physiological Perspective on its Role in Psychomotor Control. Chem. Phys. Lipids 2000, 108, 151–158; and (h) De Petrocellis, L.; Melck, D.; Bisogno, T.; Di Marzo, V. Endocannabinoids and Fatty Acid Amides in Cancer, Inflammation and Related Disorders. Chem. Phys. Lipids 2000, 108, 191–209. This broad spectrum of action makes the ECS an important therapeutic target for the treatment of diverse pathologies, including asthma, pain, multiple sclerosis, malignant gliomas, and neurodegenerative diseases. See (a) Calignano, A.; Kátona, I.; Desamaud, F.; Giuffrida, A.; La Rana, G.; Mackie, K; Freund, T. F.; Piomelli, D. Bidirectional Control of Airway Responsiveness by Endogenous Cannabinoids. Nature 2000, 408, 96–101; (b) Baker, D.; Pryce, G.; Croxford, J. L.; Brown, P.; Pertwee, R. G.; Huffman, J. W.; Layward, L. Cannabinoids Control Spasticity and Tremor in a Multiple Sclerosis Model. Nature 2000, 404, 84–87; (c) Galve-Roperh, I.; Sanchez, C.; Cortés, M. L.; Gomez del Pulgar, T.; Izquierdo, M.; Guzman, M. Antitumoral Action of Cannabinoids: Involvement of Sustained Ceramide Accumulation and Extracellular Signal-Regulated Kinase Activation. Nat. Med. 2000, 6, 313–319; and (d) Pertwee, R. G. Pharmacology of Cannabinoid Receptor Ligands. Curr. Med. Chem. 1999, 6, 635–664.
Moreover, an increased level of endocannabinoids in mammalian cells can be obtained by inhibiting their uptake and/or degradation, raising the possibility of producing local cannabimimetic effects without directly activating cannabinoid receptors with classic agonists, thereby avoiding their associated undesirable side effects. Therefore, synthetic inhibitors may be of potential therapeutic value for the treatment of disorders characterized by a low endocannabinoid activity and where direct agonists have proven to be effective, yet produce undesirable effects. Piomelli, D.; Giuffrida, A.; Calignano, A; Rodriguez de Fonseca, F. The Endocannabinoid System as a Target for Therapeutic Drugs. Trends Pharmacol. Sci. 2000, 21, 218–224. In particular, the therapeutic utility of such uptake inhibitors has been considered for the treatment of diverse pathologies as Huntington's chorea or multiple sclerosis . Baker, D.; Pryce, G.; Croxford, J. L.; Brown, P.; Pertwee, R. G.; Makriyannis, A.; Khanolkar, A.; Layward, L.; Fezza, F.; Bisogno, T; Di Marzo, V. Endocannabinoids Control Spasticity in a Multiple Sclerosis Model. FASEB J. 2001, 15, 300–302.
Anandamide
Generally, cannabinoid agonists include both exogenous active molecules as well as endocannabinoids. Exogenous agonists are usually classified as classical cannabinoids (Cannabis sativa derived compounds as, for example, Δ9-THC and their analogues), nonclassical cannabinoids (which lack the characteristic tricyclic structure of classical ones, as, for instance, CP55940), and aminoalkylindoles (e.g., WIN552122), whereas endogenous cannabinoids belong to the eicosanoid class. Among the antagonists, diarylpyrazoles merit special mention as being the most widely used compounds. Pertwee, R. G. Cannabinoid Receptor Ligands: Clinical and Neuropharmacological Considerations, Relevant to Future Drug Discovery and Development. Expert Opin. Invest. Drugs 2000, 9, 1–19.
Anandamide (arachidonylethanolamide) is an endogenous lipid that activates brain cannabinoid receptors and mimics the pharmacological effects of Δ9-tetrahydrocannabinol, the active principle of hashish and marijuana. W. A. Devane et at., Science 258, 1946 (1992); and R. Mechoulam, L. Hanus, B. R. Martin, Biochem. Pharmacol. 48, 1537 (1994). In humans, such effects include euphoria, calmness, dream states, and drowsiness. W. L. Dewey, Pharmacol. Rev. 38, 151 (1986). Depolarized neurons release anandamide through a mechanism that may require the calcium-dependent cleavage of a phospholipid precursor in neuronal membranes. V. Di Marzo et al., Nature 372, 686 (1994); and H. Cadas, S. GaiUet, M. Bettramo, L. Venance, D. Piomelli, J. Neurosci. 16, 3934 (1996); T. Sugiura et al., Eur. J Biochem. 240, 53 (1996); and H. Cadas, E. di Tomaso, D. Piomelli, J. Neurosci., 17, 1226 (1997). Moreover, anandamide may act as the chief component of a novel system involved in the control of cognition and emotion. In fact, physiological experiments have shown that anandamide may be as important in regulating our brain functions in health and disease as other better-understood neurotransmitters, such as dopamine and serotonin.
Anandamide is released from membrane compartments in neurons in response to receptor stimulation. Notably, D2 agonism stimulates anandamide release. In studies of rat brain neurons, anandamide was determined to be released by a unique mechanism: it is stored in the cell membrane in the form of a phospholipid precursor, which is cleaved by a calcium—and activity-dependent enzymatic reaction. N-arachidonoyl phosphatidylethanolamine (NAPE) has been identified as a precursor for anandamide, which is formed by a phosphodiesterase-mediated cleavage of NAPE. The biosynthesis of NAPE is catalyzed by an N-acyltransferase enzyme, which has been characterized and partially purified from rat brain extracts. The formation of NAPE and its cleavage to yield anandamide are highly regulated processes, which take place in select regions of the brain.
Like other modulatory substances, extracellular anandamide is thought to be rapidly inactivated. As outlined in the preceding section, the pathway involves hydrolysis to arachidonic acid and ethanolamine, catalyzed by a membrane-bound fatty acid amide hydrolase (FAAH) highly expressed in rat brain and liver. D. G. Deutsch and S. Chin, Biochem. Pharmacol. 46, 791 (1993); F. Desamaud, H. Cadas, O. Piomelli, J. Biol. Chem. 270, 6030 (1995). Nevertheless, the low FAAH activity found in brain plasma membranes indicates that this enzyme may be intracellular, a possibility that is further supported by sequence analysis of rat FAAH. B. Cravatt et al., Nature 384, 83 (1996). Although anandamide could gain access to FAAH by passive diffusion, the transfer rate by this mechanism is expected to be low due to the molecular size of this lipid mediator. W. D. Stein, Channels and Pumps. An Introduction to Membrane Transport, (Academic Press, San Diego, 1990), pp. 53–57. Other lipids, including polyunsaturated fatty acids and prostaglandin E2 (PGE2), enter cells by carrier-mediated transport (L. Z. Bito, Nature 256, 1234 (1975); J. E. Schaffer and H. F. Lodish, Cell 79, 427 (1994); I. N. Bojesen and E. Bojesen, Acta PhysioL Scand. 156, 501 (1996); N. Kanai et al., Science 268, 866 (1995)). As mentioned above, a rapid, saturable process of anandamide accumulation, via the anandamide transporter, into neural cells has been reported. V. Di Marzo et al., Nature 372, 686 (1994).
The inactivation of anandamide, necessary to terminate its biological effects, occurs in two steps. It is first removed from the extracellular space by a selective carrier protein that transports it into cells, where it is then broken down by hydrolysis, catalyzed by the enzyme anandamide amidohydrolase, into biologically inactive compounds. A potent inhibitor of this enzyme has been identified (a bromoenol lactone, BTNP), and its availability will facilitate pharmacological analysis of anandamide action. A high-affinity anandamide transporter has been characterized in rat cortical neurons and in astrocytes. A compound (N-(4-hydroxyphenyl)arachidonylamide) has been found that selectively and potently inhibits such transport, without binding to cannabinoid receptors or affecting anandamide hydrolysis. This transport system appears to constitute a novel lipid uptake system analogous to, but distinct from, the prostaglandin uptake system. Also, the use of these inhibitors allowed the demonstration that anandamide transport constitutes the rate-limiting step in the biological inactivation of anandamide, both in vitro and in vivo. It is important to understand how anandamide levels are regulated, because a deregulation may lead to brain dysfunction.
Anandamide and dopamine appear to act in opposite ways to control movements in an area of the brain called the dorsal striatum; dopamine stimulates movements by acting in this area, and anandamide apparently inhibits this action of dopamine. The determination that anandamide can counteract dopamine will prove useful in the development of medications for treating diseases that seem to involve dopamine inbalances in the brain. Certain diseases appear to be caused by too much dopamine in certain brain regions, or perhaps hypersensitivity of brain sites targeted by dopamine. These diseases include schizophrenia and Gilles de la Tourette syndrome, which is characterized by facial tics, repeating of words and phrases, and uncontrollable shouting of obscenities. Medications that mimic anandamide might reduce the symptoms of these and other diseases by dampening dopamine overactivity. Additionally, medications that block anandamide action in the brain should also prove useful in treating diseases that appear to be associated with too little dopamine in certain brain regions, or hyposensitivity of dopamine targets. These diseases include drug addiction and Parkinson's disease.
In rats, AM404, an AT inhibitor, prolongs the lifetime of released anandamide in the brain and reduces the psychomotor effects of dopamine D2 agonism. Painful stimuli in rats causes anandamide release that mediates a natural analgesic response in the dorsal lateral periaqueductal gray region of the brain via agonism of CB1 receptors. In various in vivo models, AM404 produced mild, slowly developing hypokinesia that was reversible by the cannabinoid CB1 receptor antagonist SR-141716A. AM404 also prevented apomorphine-induced yawning in a dose dependent manner; this effect was likewise reversed by SR-141716A. Moreover, AM404 decreased the motor behavior stimulation induced by quinpirole, a selective dopamine D2 agonist, and reduced hyperactivity in a rat model of ADHD. AM404 inhibits AT (IC50˜2 μM), but is not suitable for drug candidacy due to its low potency and specificity. The latter characteristic is likely due to its arachinonyl moeity. Additionally, using in vitro assays, researchers have shown that phenylmethylsulfonyl fluoride (PMSF) can inhibit the degradation of anandamide. Further, a series of fatty acid sulfonyl fluorides have been identified that inhibit amidase and are more potent and selective than PMSF. Deutsch, D.G. et al. “Fatty acid sulfonyl fluorides inhibit anandamide metabolism and bind to the cannabinoid receptor” Biochemical and Biophysical Research Communications 1997, 231, 217–221.
Interestingly, anandamide and structurally related lipids have recently been reported to modulate the activity of vanilloid receptors on primary sensory nerves. U.S. Patent Application Publication No. US 2002/0019444 A1. This discovery has numerous implications in the medical, pharmaceutical, and scientific fields, and provides a molecular mechanism for the non-CB1 receptor-mediated vasodilator action of anandamide. The vanilloid receptor (VR1), which was recently cloned by Caterina et al. (Caterina, M. J. et al., The capsaicin receptor: a heat-activated ion channel in the pain pathway., Nature 389, 816–824 (1997)), is a capsaicin-sensitive, heat-gated, non-selective cation channel. The work by Caterina et al. and subsequent studies have confirmed that VR1 is uniquely expressed in a subset of primary sensory neurons (Tominaga K, Caterina M J, Mahnberg A B, Rosen T A, Gilbert H, Skinner K, Raumann B E, Basbaum A I, Julius D., The cloned capsaicin receptor integrates multiple pain-producing stimuli., Neuron 21, 531–543 (1998)), which are widely distributed in the humans and animals (Holzer P., Capsaicin: cellular targets, mechanisms of action, and selectivity for thin sensory neurons, Pharmacol Rev 43, 143–201 (1991)).